1. Technical Field
The present invention relates to electrode structures and optical semiconductor elements.
2. Related Art
A surface-emitting type semiconductor laser is one type of optical elements that emit laser light. The surface-emitting type semiconductor laser is provided with a laser resonator formed in a direction orthogonal to a surface of the substrate, and emits laser light from an upper surface of the laser resonator. In the surface-emitting type semiconductor laser, surfaces around the columnar laser resonator provided on the surface of the substrate may be coated and planarized with an insulation material such as polyimide. For example, Japanese Laid-open patent application JP-A-2004-288971 is an example of related art.
A structure of a surface-emitting type semiconductor laser is described below with reference to FIGS. 15A and 15B. As shown in FIG. 15A, a laser resonator 102 is formed on an upper surface of a semiconductor substrate 101. An insulation layer 103 composed of insulation material, such as, for example, polyimide resin is provided to cover surfaces around and on an upper surface along its marginal area of the laser resonator 102. In other words, the insulation layer 103 generally forms a hill configuration having a gently sloped section 103a in the region near the marginal area of the laser resonator 102, and planarized gradually with a gentle slope as it extends away from the laser resonator 102.
Further, a ring-shaped electrode 104 is formed on the upper surface of the laser resonator 102 and the insulation layer 103. The ring-shaped electrode 104 is formed such that its end section extends to an intermediate point in the sloped surface section 103a (in a sloped surface of the hill on the opposite side of the laser resonator 102). It is noted that the ring-shaped electrode 104 is in contact with and conductively connected to the upper surface of the laser resonator 102. Also, laser light emitted from the upper surface of the laser resonator 102 passes through an opening section 104a formed in the center of the ring-shaped electrode 104 and is emitted outside. The greater the thickness of the ring-shaped electrode 104 near the opening section 104a, the more the mode of laser light emitted outside is affected. Therefore the ring-shaped electrode 104 needs to be made thinner. On the other hand, in order to inject a current effectively in the laser resonator by reducing the electrical resistance of the ring-shaped electrode 104, the ring-shaped electrode 104 may need to be made thicker. Instead of making the ring-shaped electrode 104 thicker, a thick lead-out electrode 105 is formed on the ring-shaped electrode 104 and the insulation layer 103. The lead-out electrode 105 connects the ring-shaped electrode 104 to a pad electrode (not shown) for applying a driving signal to the laser resonator 102.
When forming the ring-shaped electrode 104 and the lead-out electrode 105, a certain method is used in which the ring-shaped electrode 104 is first formed in appropriate thickness and area, and then the lead-out electrode is formed while avoiding the opening section. Also, each of the ring-shaped electrode and the lead-out electrode is formed through forming a metal film by a vapor deposition method on a resist layer having a predetermined opening configuration, and removing unnecessary portions of the metal film together with the resist by a lift-off method.
It is noted that the optical semiconductor element described above still has the following problems. When the ring-shaped electrode is formed by a lift-off method in a manner that the end section thereof reaches an intermediate point on the slope of the insulation layer on the side away from the laser resonator, the metal film may form a burr at an end of the ring-shaped electrode; and when the lead-out electrode is formed, the lead-out electrode may be disconnected at a position where it overlaps the one end of the ring-shaped electrode (see FIG. 15B). It is believed that, because the end of the ring-shaped electrode has an inversely tapered shape due to the burr, the portion in the inversely tapered shape functions as a mask when the lead-out electrode is formed, thereby causing the disconnection defect. For this reason, disconnection of lead-out electrodes causes to lower the yield in the manufacturing process.